Field of the Invention
The present invention relates to improvements in a stacked film capacitor formed by using a stack having a structure in which at least one resin dielectric film and a plurality of vapor-deposited metal films are alternately superposed on each other.
Description of Related Art
A capacitor has been used in various kinds of electronic devices and electric devices. In recent years, a film capacitor is widely used in the electronic devices and the electric devices to meet a demand for downsizing of those devices.
The film capacitor has various structures. As a kind of the film capacitor, a stacked film capacitor disclosed in JP-A-9-153434, for example, is known. This film capacitor is formed by using a stack obtained by superposing metallized films on each other. Each metallized film is constituted by a dielectric film in the form of a resin film and vapor-deposited metal film(s) provided on one or both of opposite major surfaces of the resin film. The metallized films are superposed on each other such that the resin films and the vapor-deposited metal films are alternately arranged. Protective films are superposed on respective opposite surfaces of the thus obtained stack, which surfaces are opposite to each other in a direction of superposition of the metallized films, whereby a film capacitor element is obtained. On the other hand, external electrodes are formed on respective two side surfaces of the film capacitor element, which side surfaces are opposite to each other in a direction perpendicular to the direction of superposition of the metallized films, whereby the film capacitor is obtained.
Further, JP-A-2011-181885, for example, proposes a film capacitor formed by using a stack of vapor-deposited metal films and dielectric films in the form of vapor-deposited polymer films each of which can be formed with a thickness on the order of nanometer, and superposing protective films on respective opposite surfaces of the stack, thereby forming a film capacitor element, and forming external electrodes on two opposite side surfaces of the film capacitor element. This film capacitor can be made with a smaller size than the above-described film capacitor.
Namely, the conventional stacked film capacitor is generally formed by using a film capacitor element consisting of a stack in which at least one resin dielectric film and a plurality of vapor-deposited metal films are alternately superposed on each other, and protective films on respective opposite surfaces of the film capacitor element, which surfaces are opposite to each other in the direction of superposition of the at least one dielectric film and the vapor-deposited metal films, and forming external electrodes on a pair of side surfaces of the film capacitor element.
By the way, in the conventional stacked film capacitor described above, two side surfaces of the film capacitor other than the two side surfaces on which the external electrodes are formed, namely, the two side surfaces adjacent to the latter two side surfaces are exposed to the outside of the film capacitor. Accordingly, where the stacked film capacitor is used in such a state, a large leakage current might be generated from those exposed side surfaces, and a gas such as a water vapor might enter the inside of the film capacitor through those side surfaces, leading to deterioration of the dielectric films and the vapor-deposited metal films. As a result, there arises a risk of reduction of the performance of the film capacitor. Further, in recent years, the thickness of the dielectric films is reduced to reduce the size of the stacked film capacitor. Therefore, in such a stacked film capacitor, a creepage distance between the adjacent two vapor-deposited metal films disposed on the respective opposite sides of each dielectric film is reduced, so that electricity is easily conducted between the adjacent two vapor-deposited metal films, resulting in a tendency of reduction of the withstand voltage of the film capacitor.
In order to prevent the above-described problems, there has been taken measures for insulating the side surfaces of the film capacitor adjacent to the side surfaces on which the external electrodes are formed, from the outside of the film capacitor, by accommodating the film capacitor in a casing, and filling a space between the film capacitor and inner surfaces of the casing with a resin such as an epoxy resin having an electrical insulation property, as disclosed in JP-A-2003-338424, for example. However, the film capacitor accommodated within the casing together with the resin, namely, a so-called cased film capacitor has a larger size due to the presence of the casing, and inevitably requires an extra cost.
Under the circumstances described above, JP-B-5-63094 discloses a film capacitor in which cover films are formed on the two side surfaces of the film capacitor element (stack of the metallized films) other than the two side surfaces on which the external electrodes are formed, such that entireties of the former two side surfaces are covered by the cover films. Where the film capacitor is constructed as described above, the side surfaces of the film capacitor adjacent to the side surfaces on which the external electrodes are formed can be insulated from the outside of the film capacitor, without accommodating the film capacitor within the casing.
However, the inventors of the present invention made various studies on the above-described so-called covered film capacitor obtained by forming the cover films on its exposed side surfaces, and found that the covered film capacitor has inherent problems as described below.
Namely, in the conventional covered film capacitor, the dielectric films are formed of a thermoplastic resin such as polyethylene terephthalate, whereas the cover films are formed of an epoxy-based thermosetting resin which is completely different from the resin of the dielectric films. Accordingly, sufficiently high adhesiveness between the cover films and end faces of of the dielectric films defining parts of the side surfaces of the film capacitor covered by the cover films cannot be assured, so that small gaps are inevitably formed between the cover films and the above-described end faces of the dielectric films. Therefore, in the conventional covered film capacitor, the water vapor, the air and the like enter the small gaps formed between the cover films and the above-described end faces of the dielectric films, resulting in extreme difficulties in surely preventing the problem of deterioration of the dielectric films and the vapor-deposited metal films due to their contact with the water vapor, and the problem of reduction of the withstand voltage of the film capacitor due to electric discharge between the vapor-deposited metal films.